This application claims the priority of German patent application number DE 19900324 C1, filed Jan. 7, 1999, which is incorporated by reference.
The invention relates to a data transmission method, particularly in electric networks, in which coded data are blockwise modulated onto several carrier frequencies within one or several particular frequency bands by an orthogonal frequency division multiplex method (OFDM) and in which the signal corresponding to a data block and obtained by inverse Fourier transformation is transferred to a receiver for a predetermined time duration (T), where individual signals succeed as signal blocks in time.
Such methods are used in the power line transmission technique, for example. This attempts by suitable coding and modulation of the data to be transferred to transfer them unidirectionally and bidirectionally over the DC network to receivers. To this end in Europe the bands A to D are available according to the standard CENELEC EN 50065, the bands covering the following frequency ranges: 9-95, 95-125, 125-140, and 140-148,5 kHz, respectively. In the USA or in Japan frequency ranges of up to 500 kHz are available. Because almost all households and buildings are connected to electric networks, a data transmission by power lines is desirable and obviates the additional installation of two-wire lines, coaxial cables and optical fibres. Electric equipments like central heatings, and air conditioning and lighting equipments or machines can automatically be operated, controlled and monitored by switches, sensors and actuators, for example. To this end the existing electric network for energy transfer at 50 Hz is used to transmit high frequency signals. In this respect the different characteristics of the lines at the different high frequencies, the line drops and the locally varying characteristics of the transmission medium are problematic. Within the allowed bands of the European standard the input impedance for the fed signals increases with decreasing frequency. Another difficulty results from the variety of spurious signals in the electric networks. Particularly at low frequencies a strong large bandwidth noise exists. Television sets, computers and radio transmitters cause narrow bandwidth interferences with high power density. Additionally pulse interferences occur, which are caused by power-on and power-off operations and have peak voltages in the kV range. Their interference spectrum extends far into the MHz range because of the steep leading and trailing edges.
Transmission systems for electric networks with data rates on the order of several Mbit/s using bandwidths of several MHz are presently investigated. However, a high data transmission rate employing the frequency bands of the European standard is desirable.
For telecommunications over energy distribution networks a method is known (Intellon Corporation, 1998, White Paper #0032, IeOFDM High Speed No New Wires, Revision 2.0, http://www.intellon.com), in which data are transferred over AC lines by a well known orthogonal frequency division multiplex method. To this end the available frequency spectrum is divided into many narrow bandwidth areas, in which data are respectively coded by phase modulation of a carrier frequency. The OFDM modulation is generated by a fast Fourier transform (FFT) processor, the time signal being obtained by subsequent inverse Fourier transformation. By copying a signal portion of a few microseconds length located at the end of the signal to its beginning the signal block to be transferred is generated. The length of the copied portion depends upon the reflection period of the original signal in the transmission channel, respectively. The copy operation is designed to avoid an interference of the original signal block with its time delayed reflections. The achieved transmission rates should be 10 Mbit/s in a frequency band above 2 MHz.
It is the copy operation that proves to be disadvantageous with this well-known telecommunication method, because it leads to a discontinuous transition at the connection point, which is also the case, when the individual signal blocks are lined up. These discontinuities in the time range result in components in the frequency range, which there can overlap other carrier frequencies and which can cause interferences outside the allowed frequency band. Moreover, the reflection duration in the transmission channels and thereby the length of the signal portion to be copied varies.
The object of the present invention is to provide a data transmission method, especially for electric networks, which operates in narrow frequency bands with a high data transmission rate without causing interferences outside the used frequency band. Especially country specific standards have to be respected.
Preferred embodiments are set out in the subclaims.
According to the invention each signal is multiplied by a window function before transmission, the thus generated signal blocks being transferred successively in time. The window function mustn""t cut off the signal at the margins, because discontinuities have to be avoided as far as possible, if high frequency interference components are to be eliminated. Therefore according to the invention a window function has to be chosen, the absolute value of the Fourier transform of which having the following characteristics: at the used carrier frequencies respective zero crossings have to exist and the secondary maxima have to have a damping of at least xe2x88x9230 dB in relation to the main maximum (at the frequency 0). The first condition guaranties that there is an orthogonality for the used carrier frequencies, which means that the resulting signal, which represents a linear combination of harmonic functions distributed among the carrier frequencies is composed without mutual interference of, the respective functions and hence carriers and can thus be decomposed uniquely into the individual carrier components. The second condition guarantees that outside the carrier frequency range a fast decrease of the power spectrum takes place. This especially allows the fulfillment of mandatory standards when using particular frequency ranges in electric networks.
The joining of signal blocks of specific time duration in the time range corresponds to the multiplication of a signal with a square window of time duration T, the resulting signal blocks subsequently being joined in time. The multiplication in the time range corresponds to a convolution in the frequency range. The interferences caused in the frequency range are the smaller, the wider the square window is chosen in the time range. In the well-known Intellon method the copying of a narrow time signal portion therefore results in broadband interferences.
A square window of time duration T has respective zero crossings at multiples of the value 1/(2xc3x97T), at which places the carrier frequencies can be placed without overlapping of information of other carriers. However it remains disadvantageous that the high frequency components generated by the square window are damped only very slowly. This results in the above-mentioned spurious frequencies outside the used frequency band and thus to an incompatibility with the standard.
Therefore according to the invention a window has to be used, which has as many zero crossings as possible within the allowed frequency band, to avoid limiting the number of carriers, and which also has a strong damping, which has to be at least xe2x88x9230 dB outside the main maximum. To fulfil these requirements, even only every second of the interesting carrier frequencies in a square window can be used, if the damping condition can be fulfilled thereby. Especially desired are dampings which already from the second secondary maximum on are at least xe2x88x9250 to xe2x88x9260 dB.
A possible window function can be represented by the following formula:       window    ⁢          xe2x80x83        ⁢          (      n      )        =            c      0        +                  ∑                  i          =          1                I            ⁢              xe2x80x83            ⁢                        c          i                ⁢                  xe2x80x83                ⁢        cos        ⁢                  xe2x80x83                ⁢                  (                                    (                                                2                  ⁢                  i                                -                1                            )                        ⁢                          xe2x80x83                        ⁢            2            ⁢                          xe2x80x83                        ⁢            π            ⁢                          xe2x80x83                        ⁢                                          (                                  n                  +                  0.5                                )                            NFFT                                )                    
Here NFFT represents the number of points used for the inverse Fourier transformation in the signal generation. n runs from 0 to (NFFT-1) corresponding to the time interval from t0 to t0+T. The coefficients ci can be chosen so that the Fourier transform of this function decreases as rapidly as possible and has already at the second secondary maximum a damping of at least xe2x88x9250 dB. For I=2 these coefficients can be represented as follows:
c0=0.5093
c1=xe2x88x920.4941
c2=0.0059
This window (illustrated in FIG. 3) satisfies the orthogonality condition at least at every second carrier frequency (in comparison to the square window). The damping condition is also satisfied with already the second secondary maximum having decreased to xe2x88x9259 dB.
In the data transmission method according to the invention the data are preferably modulated by quadrature amplitude modulation (QAM) in combination with OFDM. Here 2, 4, 8, 64, 128, 256 etc. pieces of information can be coded by phase and/or amplitude modulation with 8 bits per carrier being coded in a 256-QAM. In this respect it is favourable to distribute the information equidistantly upon neighbouring squares in the complex space, the size of which is predetermined by the phase and amplitude uncertainty of the transmission path.
The method according to the invention can particularly be used for the transfer of data in the B and D bands of the Cenelec standard. The signals can then be distributed in the low voltage distribution network, for example, with an xe2x80x9cintra-buildingxe2x80x9d data transmission being especially favourable.
It is favourable to generate the signal by a digital signal processor in the frequency space, to transform it by inverse Fourier transformation into the time space where it is multiplied by the time window according to the invention and successively converted by a digital/analog converter so that it can be fed into the transmission carrier. This method allows a high transmission rate because of the very rapidly performable Fourier transformations and thus telecommunications in real-time.
Because the generated signal blocks can contain little energy at the beginning and the end, they can be overlapped in order to increase the number of blocks per second. This way a data transmission rate comparable to the ISDN rate can easily be achieved.
The method according to the invention can also be used for the bidirectional data transmission apart from the unidirectional data transmission by dividing the number of carrier frequencies into a transmission channel and a return channel. Also favourable is another synchronisation channel for which a carrier frequency can be reserved. The synchronisation channel allows the permanent synchronisation of several stations without a minor quartz-drift leading to big deviations in the long term.
In accordance with the type of the data transmission it may be necessary to transfer data in real-time, i.e. without delays (e.g. in telecommunications) or to transmit the data reliably and error free in binary form with delays being permissible. For error correction the overall data to be transmitted can be distributed into two packages with one number of bits being used as control bits. In this respect the use of BCH codes is preferable.
The data transmission rate can be increased by using a transmission and a return channel, if both channels have the same carrier frequencies. The requirement for this is a transmission path with low reflections.
Another possibility of data transmission is to feed each signal through a digital filter before transmission, the transfer function of the filter having a damping of at least xe2x88x9230 dB outside the respectively used frequency bands with all carriers within the allowed bandsxe2x80x94instead of every second one in the window techniquexe2x80x94being usable, whereby the data rate is doubled. The digital filter operates with a linear phase, i.e. it has symmetric coefficients (cxe2x88x92k=Ck) and an odd number of coefficients (k=xe2x88x92m, . . . , 0, . . . m). Because the signal block is prolonged in time by the use of a digital filter, the individual blocks mustn""t be transferred directly succeeding in time. It proves to be advantageous to choose about 10% of the block length as time separation between the signal blocks. In the selection of the filter coefficients the damping condition, i.e. a strong damping outside the used frequency bands, and the orthogonality condition, i.e. the avoidance of a mutual interference of the carriers, is to be respected.
In comparing both data transmission methods according to the invention by employing a window function and by digital filtering of the respective signal blocks, respectively, the following can be noted: the digital filtering technique requires more computational overhead than the window technique, and moreover the orthogonality requirement can not entirely be satisfied. On the other hand twice the number of carrier frequencies can be used and even at low signal-to-noise ratios on the transmission path a reliable data transmission can be realised. Finally the signal blocks can be overlapped in time in the window technique, whereas they are prolonged in time in the digital filtering technique.